1. Field of the Invention
This invention relates to methods and apparatus for use in piston internal combustion engines for conditioning the inlet airstream and for conditioning fuel discharged into the inlet airstream.
2. Description of the Prior Art
In the operation of gasoline-fuelled piston internal combustion engines, it is normal practice for fuel to be discharged into the inlet airstream in the form of finely divided liquid droplets. Droplet delivery has the advantage that fuel in this form displaces a minimum of air, thereby making available a maximum of oxygen for combustion of the fuel in the engine cylinder. While some evaporation of the droplet fuel does occur during passage of the mixture through the inlet tract and further evaporation occurs as a result of the temperature rise associated with compression of the charge in the cylinder, part of the fuel may remain in liquid droplet form at the onset of ignition. As ignition is initiated in the vapour content of the charge, it is therefore necessary to provide a greater quantity of fuel in the charge in order to ensure a sufficiently rich vapour mixture for ignition to reliably occur under all operating conditions. In conventionally carburetted engines, lean mixtures are provided for low power operation and additional fuel is released through power enrichment provisions during high BMEPs.
It has been demonstrated that, once a self-propagating flame front has been established in the cylinder, it will reliably propagate throughout the generality of a charge having a much leaner mixture strength than that required for reliable ignition to occur. Many systems have been proposed for exploiting this effect by providing a locally enriched mixture in the ignition region. Such charge stratification methods are well known in the art, but have not found favour because of the extra cost and complexity they entail. In some engines, reliability of ignition in lean mixtures has been improved by providing larger, more energetic or multiple ignition sources in each cylinder.
It is also well known in the art that, in engines in which the cylinders are supplied with mixture through a multi-branched inlet manifold, it is difficult to ensure an even distribution. As a result, some cylinders tend to receive a leaner mixture than others, making a compensatory increase in overall mixture strength necessary. Excepting where individual carburettors feed individual cylinders through straight inlet tracts, this effect is common in conventionally carburetted engines. The problem of unequal mixture distribution is not mitigated by throttle box fuel injection arrangements, which retain conventional inlet manifolding provisions.
Inequalities in mixture distribution as a result of inlet tract inadequacies have been commonly overcome by injecting fuel in droplet form directly into the opening of the inlet port, from whence it is entrained in the inlet airflow and carried into the cylinder. While port injection of a quantity of fuel accurately appropriate to the instantaneous operating parameters of an engine does mitigate the unequal mixture distribution problem, there is less opportunity for evaporation of the droplet fuel in the short travel distance involved and some will still remain in droplet form at the onset of ignition. Thus, absent charge stratification or more elaborate ignition provisions, a richer than necessary mixture strength is still required in order to ensure the presence of sufficient fuel in vapour form for the achievement of reliable ignition.
It is well known that diesel engines commonly operate at lambda figures exceeding 1.5. This means that in excess of 150% of the air required for a stoichiometric mixture is supplied to a cylinder. Such high lambda figures are a consequence of the timed injection of droplet fuel employed in the diesel engine and are largely responsible for its excellent fuel economy. In contrast, gasoline-fuelled engines of conventional arrangement seldom exceed lambda figures of 1.1, with lower figures occurring at higher powers. However, it has been shown that, in the same engines, a charge comprising dry gaseous fuels, for example, propane, thoroughly mixed with air to provide an homogenous mixture will provide reliable ignition at lambda figures of 1.3 at relatively high BMEPs and there is evidence that operation up to lambda figures of around 1.5 can readily be achieved.
While separate complete evaporation of gasoline fuel in the manner employed with liquefied petroleum gas fuel is not practical due to difficulties in vaporising certain essential additives contained in fuels such as gasoline, there is evidence that, if gasoline can be fully evaporated in the inlet airstream and thoroughly mixed with the charge air, operation at relatively high BMEPs is achievable at lambda figures similar to the best achievable with dry gaseous fuels. To achieve a high degree of evaporation of droplet fuel prior to entry of the mixture to the cylinder, heating of all or part of the charge air and, in some cases, the fuel, is necessary.
It is conventional belief that, in air breathing engines, charge temperature should be maintained as low as possible to provide the greatest possible charge density, thereby improving volumetric efficiency. It is also believed that the onset of detonation is delayed by maintaining a low charge temperature. It can be demonstrated that high charge temperatures may, in fact, be provided or tolerated with beneficial effect, with volumetric efficiency being restored in various ways and with premature detonation not proving to be a problem.
The advantages of heating fuel and inlet air to achieve substantial evaporation of fuel in the inlet tract of a gasoline-powered piston internal combustion engine have long been recognised and a variety of methods have been proposed for this purpose. Typical of these is that taught by Sviridov et al in U.S. Pat. No. 4,438,750 in which a vaporizing element extends from an exhaust port to an adjacent inlet port. A heat take-up part of the vaporizing element is heated by exhaust gases and heat captured thereby is conducted via a connecting member of the vaporizing element to its curved operating surface situated in the inlet port. Fuel is injected into the inlet port via an electromagnetically-controlled injection nozzle at a tangent to the operating surface to provide better conditions for fuel film forming. A stated object is the vaporisation of injected fuel to accomplish complete homogenisation of an air-fuel mixture before it enters the engine cylinder. Similarly, in methods taught by Scherenberg et al in U.S. Pat. No. 5,140,967 and Jordan in U.S. Pat. No. 3,930,477, a heating bar or element is provided in each inlet port and fuel discharged by a fuel injector into the heated region is subject to accelerated vaporisation. In another example taught by Yokoi et al in U.S. Pat. No. 4,483,304, rapid evaporation of fuel films is achieved by providing electrical resistance-heated fuel vaporizers at various positions within the inlet tracts of an engine. In another example taught by Swanson in U.S. Pat. No. 4,375,799, fuel vaporization means are provided in the form of a vaporization chamber embedded in one wall of a carburettor, the chamber being heated by surrounding electrical heating elements. Fuel is discharged into the vaporization chamber and evaporated before being discharged into the venturi of the carburettor. In another example taught by Oblander in U.S. Pat. No. 3,461,850, the main exhaust manifold runner is made unitary with the inlet manifold runners of an engine, resulting in a heated zone in each runner. Fuel is discharged from a conventional fuel injector onto the heated zone, the stated object being an improvement in fuel preparation. In another example taught by Gardner et al in U.S. Pat. No. 4,583,512, the carburettor or fuel injection system are deleted from an engine and the inlet air and fuel are heated by a series of electrical resistive elements in separate heat exchange units. The heated fuel and air are combined and further heated in a common heat exchange unit and admitted to the combustion chambers of the engine through a series of electrically-operated valves. A stated object, amongst others, is the provision of better fuel vaporisation and fuel to air ratio. In another example taught by Hoppie et al in U.S. Pat. No. 4,664,925, a quantity of fuel is pre-heated by passing it through a coiled tube positioned in each exhaust port. The fuel is vaporised and rapidly heated to a high temperature by being adiabatically compressed immediately prior to injection into the combustion chamber. Stated objects are to achieve negligible ignition delay and substantially instantaneous completion of the combustion process. In another example taught by Lakin in U.S. Pat. No. 6,712,051, part of the inlet air is heated by engine coolant or exhaust system waste heat. The heated air is mixed with ambient air in a computer-controlled temperature regulation system based on the power output required from the engine. A fuel injection system is employed to maintain correct fuel-air ratios over the extended range of combustion air temperatures.
Of the cited examples, the method taught by Sviridov et al requires adjacent inlet and exhaust runners, which is impractical in many engines. Additionally, the temperature of the vaporising element is uncontrolled, resulting in the possible overheating of the vaporizing element in some operating conditions. Similar objections are made to the method taught by Oblander. In the methods taught by Scherenberg et al and Jordan, the flow of inlet air over the heating bar or element would result in rapid cooling, requiring a high electrical current flow to be of any significant effect. The necessary generation of electrical current would result in an adverse effect upon the efficiency and fuel economy an engine. Similar objections are made to the method taught by Yokoi et al. The method taught by Swanson requires use of a carburettor and is therefore considered impractical in light of modern automotive practice. The method taught by Gardner et al requires a high electrical current flow to be of any significant effect and the necessary generation of electrical current would result in an adverse effect upon the efficiency and fuel economy an engine. In the method taught by Hoppie et al, it is difficult to see how cold starting would be achieved. Additionally, basic fuel-air mixture control and the production of an homogeneous charge strength would present difficulties. The method taught by Lakin would provide minimal benefits in normal operation.